Svidró, Péter

Abstract [en]

Lamellar cast iron is a very important technical alloy and the most used material in the casting production, and especially in the automotive industry which is the major consumer. Beside the many great properties, it is inclined to form casting defects of which some can be prevented, and some may be repaired subsequently. Shrinkage porosity is a randomly returning problem, which is difficult to understand and to avoid. This defect is a volumetric deficiency which appear as cavities inside the casting in connection to the casting surface. Another frequent defect is the metal expansion penetration. This defect is a material surplus squeezed to the casting surface containing sand inclusion from the mold material. Shrinkage porosity is usually mentioned together with metal expansion penetration as the formation mechanism of both defects have common roots. It is also generally agreed, that these type of defects are related to the volumetric changes occurring during solidification. Additionally, the formation of these defects are in connection with the coherency of the primary austenite dendrites.

The purpose of this work was to develop knowledge on factors affecting a volume-change related casting defect formation in order to minimize the presence of these defects in engine component production. This was done by extending the existing solidification investigation methods with novel solutions. Introduction of expansion force measurement in the determination of dendrite coherency combined with multi axial volume change measurement refine the interpretation of the solidification. Comparison of registered axial and radial linear deformation in cylindrical samples indicated an anisotropic volume change. Different methods for dendrite coherency determination have been compared. It was shown that the coherency develops over an interval. Dependent on the added inoculant the coherency is reached at different levels of fractions of a solidified primary phase. It is also shown, that inoculation has an effect on the nucleation and growth of the primary phase. Quantitative image analysis has been performed on the primary phase in special designed samples designed to provoke shrinkage porosity and metal expansion penetration. It was found, that the inter-dendritic space varies within a casting. This was explained by the coarsening of the primary dendrites which originates from differences in the local time of solidification.

Elmquist, Lennart

Dugic, Izudin

Diószegi, Attila

Abstract [en]

Shrinkage porosity and metal expansion penetration are two fundamental defects appearing at production ofcomplex shaped lamellar cast iron components. In previous work it has been shown that both shrinkageporosity and metal expansion penetration are related to the primary austenite dendrite network and itsformation mechanisms. The purpose of the present work is to study the morphology of primary austenite intest casting with a high tendency to form shrinkage porosity and metal expansion penetration. Simplified testmodels simulating the thermal and geometrical conditions similar to the conditions existing in complex shapedcasting have been successfully used to provoke shrinkage porosity and metal expansion penetration.Stereological investigation of the primary dendrite morphology indicates a maximum interdendritic space inconnection to the casting surface where the porosity and the penetration defect appear. Away from the defectformation area the interdendritic space decreases. Furthermore the local solidification times of the investigatedsamples were calculated in a 3D simulation software. Comparison of the simulated local solidification timesand measured interdendritic space indicates a strong relation of the same shape as it is known from theliterature when dynamic coarsening mechanism is characterized. The main outcome of the present paper is theobserved gradient of increasing interdendritic space from sections with high local solidification to sectionswith low solidification time. The mechanism of increasing the interdendritic phase can be explained by thedynamic ripening process. The unfortunate thermal conditions with the slowest local solidification timesituated in the border between the casting surface and its surrounding are considered the reason to form anaustenite morphology which can promote the mass flow between dendrite provoking shrinkage porosity ormetal expansion penetration.